Directly Speaking About Direct Injection

Direct injection is the coming technology for both gasoline and diesel engines, according to Bosch. How quickly it arrives depends on overcoming some technical hurdles, and convincing automakers and consumers that they can have their cake and eat it, too.

Direct injection is the wave of the future, whether the engines in question are fueled by gasoline or diesel, say engineers from Robert Bosch Corp. (www.bosch.com). "We envision a very strong push into gasoline direct injection (GDI) combined with homogenous combustion and turbocharging. These technologies can be used to downsize engines or increase torque output so cars are more fun to drive," says Bernd Bohr, chairman, Automotive Group, Robert Bosch. Diesel engines, on the other hand, are facing strict NOx and particulate standards both in Europe and the U.S., making the reduction of engine-out emissions more important than ever. "Homogenous combustion and shaping the injection of fuel wave through direct injection are two very important technologies for diesel," say Bohr.

GDI: Quickly Moving Forward

Though it burst on to the scene with promises of greatly increased fuel economy and stratified charge operation, gasoline direct injection has matured into a homogenous charge strategy with much lower NOx emissions and greater engine output than non-homogenous GDI engines. The use of a homogenous intake charge lessens the chance of detonation, making it possible to run higher compression ratios on both naturally aspirated and turbocharged engines. Says Dr. Rolf Leonhard, executive vice president, Engineering, Bosch Gasoline Systems: "It wasn't until the combination of direct injection and turbocharging that diesel the engine had its market breakthrough. I think the same will be true for the gasoline engine." The addition of variable valve timing further increases the available engine tuning parameters.

"Audi's 2.0-liter Turbo DI engine has doubled the output of its port injected 1.6-liter engine with no changes in fuel consumption or emissions," says Leonhard. Second-generation GDI systems will use high-pressure, multi-hole injectors in combination with an improved high-pressure pump and revised ECUs. "The solenoid-driven multi-hole injectors are designed for fuel pressures of 200 bar versus the previous limit of 120 bar," says Leonhard. "This allows us to adjust the spray geometry to existing combustion chamber designs, rather than redesign the engine for GDI." Bosch also has developed piezoelectric injectors with extremely short switching times to allow the maximum exploitation of injection strategies under changing conditions. The high-pressure fuel pump is a one-cylinder design driven by an additional cam on the camshaft. It is smaller and lighter than the first-generation pump, has a lower power draw than a pressure-regulated pump, and is compatible with most available fuels.

Performance versions of the joint BMW/Peugeot small four-cylinder engine (http://autofieldguide.com/articles/wip/0205wip06.html) will combine all of Dr. Leonhard's required hardware–turbocharger, direct injection, variable valve timing–to produce two high-output variants with 140 and 170 hp. There is a rumored higher-output variant–producing well over 200 hp–that could move GDI technology to its next level. Bosch is developing a "GDI with scavenging" revision that combines high output with a nearly flat torque curve. As shown in a modified Audi A4 Turbo DI, Bosch engineers increased valve overlap to 40? to scavenge the residual gas from the cylinders. The result is an engine that produces 340 Nm (250 lb-ft) of torque from 1,500 rpm to redline, and greatly increases drivetrain flexibility. This technology is anywhere from 1 to 3 years from production, and dependent on the OEM's current GDI development. This is rumored to be the technology chosen by BMW and Peugeot.

Diesel: Increasingly Homogenous

"Homogenous charge compression ignition (HCCI) for diesel engines is something we are working very hard to perfect," says Bohr, "because it has huge potential. You can reduce particulates and NOx emissions by 80% to 90% with this technology, and we are spending significant resources to develop it." Currently, it's possible to run a homogenous mixture at set points on the engine load map. Dynamically moving from partially homogenous to standard to fully homogenous combustion, and covering all of the points in-between, however, is still proving to be difficult. Especially since HCCI diesels require a high level of exhaust gas recirculation to keep the fuel from being ignited too early. "I cannot say when we will get to a production-ready design," says Bohr, "but we will get there."

The importance of HCCI technology is underlined by the European Union's proposed maximum particulate levels. "They haven't been set," says Dr. Ulrich Dohle, president, Bosch Diesel Systems, "but the proposal for 2010 would reduce particulate emission to 5 mg/km from today's level of 25 mg/km. All diesel cars would have to be fitted with particulate filters, and the safety cushion significantly increased to guarantee this level could be maintained throughout the vehicle's working life."

This will require reducing raw engine-out emissions, and the best way to do that is through tighter control of the combustion process. "It's almost impossible to inject a lower rate of fuel at a higher pressure," says Bohr, "so this means you have to pulse the injections." Bosch currently uses piezoelectric injectors in its third-generation common rail system to introduce five or more injections per cycle with a maximum injection pressure of 1,600 bar. Higher performance and heavier vehicles–those weighing 3,900 lb or more –require injection pressures of 1,800 bar. "Our next-generation common rail system uses a hydraulically amplified diesel injector that can produce injection pressure of up to 2,500 bar," says Dohle. Generating the required pressure at the injector should allow pressures within the injection rail itself to be significantly lower, thus reducing the need for specialized materials and production processes or the chance that the rail might burst. "We're also hoping to lower emissions by injecting fuel with an increasing pressure," says Dohle. This eliminates the abrupt on/off switching currently used and prevents the formation of soot and carbon deposits that are a result of incomplete combustion.

As gasoline prices stabilize at historically high levels in the U.S., Bohr expects GDI and diesel engines to gain in popularity for both light trucks and cars. "We are seeing a lot of interest from European and even some Asian manufacturers for automotive diesel applications in America," he says, "and I think you will see a few of these automakers offering some of their diesels as early as 2007." He also expects GDI use to increase among domestic automakers as they downsize some engine offerings and use the technology's greater output with no loss in fuel economy to differentiate their vehicles. "It will depend, in large part, on the effect higher gas prices have on the American consumer," says Bohr.